Method and system for distributed raid implementation

ABSTRACT

Embodiments of the systems and methods disclosed provide a distributed RAID system comprising a set of data banks. More particularly, in certain embodiments of a distributed RAID system each data bank has a set of associated storage media and executes a similar distributed RAID application. The distributed RAID applications on each of the data banks coordinate among themselves to distribute and control data flow associated with implementing a level of RAID in conjunction with data stored on the associated storage media of the data banks.

RELATED APPLICATIONS

This application claims a benefit of priority under 35 U.S.C. §119 toprovisional patent application Nos. 61/131,270 by inventors Galloway etal., entitled “Method and System for Distributed RAID Implementation”filed on Jun. 6, 2008; and 61/131,314 by inventors Galloway et al.,entitled “Method and System for Data Migration in a DistributedMulti-Processor RAID Subsystem” filed Jun. 6, 2008; and 61/131,291 byinventors Galloway et al., entitled “System and Method for DistributingRead/Write Requests to Optimal SCSI Targets” filed Jun. 6, 2008; and61/131,290 by inventors Galloway et al., entitled “Method and System forUtilizing Storage in a Storage System” filed Jun. 6, 2008; and61/131,379 by inventors Galloway et al., entitled “Method and System forRebuilding Data” filed Jun. 6, 2008; and 61/131,312 by inventorsGalloway et al., entitled “Method and System for Placement of Data onStorage” filed Jun. 6, 2008, the entire contents of each are herebyfully incorporated by reference herein for all purposes.

TECHNICAL FIELD

This invention relates generally to the use of storage devices. Moreparticularly, embodiments of this invention relate to implementing RAIDon storage devices. Even more specifically, certain embodiments of thisinvention relate to a distributed implementation of RAID.

BACKGROUND

Data represents a significant asset for many entities. Consequently,data loss, whether accidental or caused by malicious activity, can becostly in terms of wasted manpower, loss of goodwill from customers,loss of time and potential legal liability. To ensure proper protectionof data for business, legal or other purposes, many entities may desireto protect their data using a variety of techniques, including datastorage, redundancy, security, etc. These techniques may, however,conflict with other competing constraints or demands imposed by thestate or configuration of computing devices used to process or storethis data.

One method for dealing with these tensions is to implement a RedundantArray of Independent Disks (RAID). Generally, RAID systems divide andreplicate data across multiple hard disk drives (or other types ofstorage media), collectively referred to as an array, to increasereliability and in some cases improve throughput of computing devices(known as a host) using these RAID systems for storage. To a host then,a RAID array may appear as one or more monolithic storage areas. When ahost desires to communicate (read, write, etc.) with the RAID system thehost communicates as if the RAID array were a single disk. The RAIDsystem, in turn, processes these communications to implement a certainRAID level in conjunction with such communications. These RAID levelsmay be designed to achieve some desired balance between a variety oftradeoffs such as reliability, capacity, speed, etc. For example, RAID(level) 0 distributes data across several disks in a way which givesimproved speed and utilizes substantially the full capacity of thedisks, but all data on a disk will be lost if the disk fails; RAID(level) 1 uses two (or more) disks which each store the same data, sothat data is not lost so long as one disk survives. Total capacity ofthe array is substantially the capacity of a single disk and RAID(level) 5 combines three or more disks in a way that protects dataagainst loss of any one disk; the storage capacity of the array isreduced by one disk.

Current implementations of RAID may have a variety of problems. Theseproblems may stem from limitations imposed by the architecture of theseRAID systems, such as the fact that in many instances all communicationswith a RAID system must be addressed to a single server which controlsand manages the RAID system. Other problems may arise from theconfiguration or layout of the data on the disks comprising a RAIDsystem. For example, in certain cases a RAID level must be chosen andstorage allocated within the RAID system before the RAID system can beutilized. Thus, the initially chosen RAID level must be implemented inconjunction with the data stored on the RAID system, irrespective ofwhether that level of RAID is desired or needed. In many cases theseexisting problems may be exacerbated by the need to use custom hardwareor software to implement these solutions, raising the costs associatedwith implementing such a solution.

Consequently, it is desired to substantially ameliorate these problems.

SUMMARY

Embodiments of the systems and methods disclosed provide a distributedRAID system comprising a set of data banks. More particularly, incertain embodiments of a distributed RAID system each data bank has aset of associated storage media and executes a similar distributed RAIDapplication. The distributed RAID applications on each of the data bankscoordinate among themselves to distribute and control data flowassociated with implementing a level of RAID in conjunction with datastored on the associated storage media of the data banks.

Specifically, in one embodiment, a volume with an associated RAID levelmay be created using the distributed RAID system. Each of thedistributed RAID applications can then coordinate operations associatedwith data of that volume such that data associated with that volume orthe implementation of the desired RAID level in conjunction with thatvolume may be stored on the multiple data banks of the distributed RAIDsystem.

By coordinating the implementation of a level of RAID in conjunctionwith a volume by storing both data of the volume and data associatedwith the implementation of RAID on multiple data banks using similardistributed RAID applications executing on each of those data banks anumber of advantages may be achieved. Namely, different storage volumesmay be allotted, with one or more of the volumes implemented inconjunction with different RAID levels. Moreover, as the coordination ofstorage and the implementation of RAID across the data banks isaccomplished using substantially identical distributed RAIDapplications, in many cases standard or off-the-shelf hardware, such asstandard x86 based servers and storage media may be utilized.

Furthermore, by distributing control across each of the data banks ofthe RAID system, each of the distributed RAID applications on each databank can execute substantially autonomously. Additionally, some degreeof fault tolerance may be inherent in his architecture as one data bankcan be lost and the RAID system may still be able to operate in aseamless manner with respect to each of the hosts utilizing the RAIDsystem.

Moreover, as a side effect of embodiments of systems and methodspresented herein, improved performance may be achieved as there arefewer performance bottlenecks, increased bandwidth may be available aseach host may be coupled to a switch and each data bank coupled to thatswitch and expansion or contraction of such a distributed RAID systemmay be accomplished relatively seamlessly.

These, and other, aspects of the invention will be better appreciatedand understood when considered in conjunction with the followingdescription and the accompanying drawings. The following description,while indicating various embodiments of the invention and numerousspecific details thereof, is given by way of illustration and not oflimitation. Many substitutions, modifications, additions orrearrangements may be made within the scope of the invention, and theinvention includes all such substitutions, modifications, additions orrearrangements.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings accompanying and forming part of this specification areincluded to depict certain aspects of the invention. A clearerimpression of the invention, and of the components and operation ofsystems provided with the invention, will become more readily apparentby referring to the exemplary, and therefore nonlimiting, embodimentsillustrated in the drawings, wherein identical reference numeralsdesignate the same components. Note that the features illustrated in thedrawings are not necessarily drawn to scale.

FIG. 1 is a block diagram of one embodiment of an architecture employinga distributed RAID system.

FIG. 2A is a block diagram of one embodiment of a data bank.

FIG. 2B is a block diagram of one embodiment of an architecture for adata bank.

FIG. 3 is a flow diagram of one embodiment of a method implemented by adistributed RAID system.

FIG. 4 is block diagram of an example of one embodiment of anarchitecture employing a distributed RAID system.

FIG. 5 is a block diagram of one embodiment of a table.

FIG. 6 is a block diagram of one embodiment of a table.

FIG. 7 is a block diagram of one embodiment of a table.

FIG. 8 is a flow diagram of one embodiment of a method implemented by adistributed RAID system.

FIG. 9A is a flow diagram of one embodiment of a method implemented by adistributed RAID system.

FIG. 9B is a flow diagram of one embodiment of a method implemented by adistributed RAID system.

FIG. 10 is a block diagram of one embodiment of a write cache.

FIG. 11 is a flow diagram of one embodiment of a method implemented by adistributed RAID system.

FIG. 12 is a flow diagram of one embodiment of a method implemented by adistributed RAID system.

DETAILED DESCRIPTION

The invention and the various features and advantageous details thereofare explained more fully with reference to the nonlimiting embodimentsthat are illustrated in the accompanying drawings and detailed in thefollowing description. Descriptions of well known starting materials,processing techniques, components and equipment are omitted so as not tounnecessarily obscure the invention in detail. It should be understood,however, that the detailed description and the specific examples, whileindicating preferred embodiments of the invention, are given by way ofillustration only and not by way of limitation. Various substitutions,modifications, additions and/or rearrangements within the spirit and/orscope of the underlying inventive concept will become apparent to thoseskilled in the art from this disclosure. Embodiments discussed hereincan be implemented in suitable computer-executable instructions that mayreside on a computer readable medium (e.g., a HD), hardware circuitry orthe like, or any combination.

Before discussing specific embodiments, embodiments of a hardwarearchitecture for implementing certain embodiments is described herein.One embodiment can include one or more computers communicatively coupledto a network. As is known to those skilled in the art, the computer caninclude a central processing unit (“CPU”), at least one read-only memory(“ROM”), at least one random access memory (“RAM”), at least one harddrive (“HD”), and one or more input/output (“I/O”) device(s). The I/Odevices can include a keyboard, monitor, printer, electronic pointingdevice (such as a mouse, trackball, stylist, etc.), or the like. Invarious embodiments, the computer has access to at least one databaseover the network.

ROM, RAM, and HD are computer memories for storing computer-executableinstructions executable by the CPU. Within this disclosure, the term“computer-readable medium” is not limited to ROM, RAM, and HD and caninclude any type of data storage medium that can be read by a processor.In some embodiments, a computer-readable medium may refer to a datacartridge, a data backup magnetic tape, a floppy diskette, a flashmemory drive, an optical data storage drive, a CD-ROM, ROM, RAM, HD, orthe like.

At least portions of the functionalities or processes described hereincan be implemented in suitable computer-executable instructions. Thecomputer-executable instructions may be stored as software codecomponents or modules on one or more computer readable media (such asnon-volatile memories, volatile memories, DASD arrays, magnetic tapes,floppy diskettes, hard drives, optical storage devices, etc. or anyother appropriate computer-readable medium or storage device). In oneembodiment, the computer-executable instructions may include lines ofcomplied C++, Java, HTML, or any other programming or scripting code.

Additionally, the functions of the disclosed embodiments may beimplemented on one computer or shared/distributed among two or morecomputers in or across a network. Communications between computersimplementing embodiments can be accomplished using any electronic,optical, radio frequency signals, or other suitable methods and tools ofcommunication in compliance with known network protocols.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,process, article, or apparatus that comprises a list of elements is notnecessarily limited only those elements but may include other elementsnot expressly listed or inherent to such process, process, article, orapparatus. Further, unless expressly stated to the contrary, “or” refersto an inclusive or and not to an exclusive or. For example, a conditionA or B is satisfied by any one of the following: A is true (or present)and B is false (or not present), A is false (or not present) and B istrue (or present), and both A and B are true (or present).

Additionally, any examples or illustrations given herein are not to beregarded in any way as restrictions on, limits to, or expressdefinitions of, any term or terms with which they are utilized. Instead,these examples or illustrations are to be regarded as being describedwith respect to one particular embodiment and as illustrative only.Those of ordinary skill in the art will appreciate that any term orterms with which these examples or illustrations are utilized willencompass other embodiments which may or may not be given therewith orelsewhere in the specification and all such embodiments are intended tobe included within the scope of that term or terms. Language designatingsuch nonlimiting examples and illustrations includes, but is not limitedto: “for example”, “for instance”, “e.g.”, “in one embodiment”.

This application is related to U.S. patent application No. 12/479,360,entitled “Method and System for Data Migration in a Distributed RAIDImplementation” by Galloway et al., filed on Jun. 5, 2009; patentapplication No. 12/479,403, entitled “Method and System for DistributingCommands to Targets” by Galloway et al., filed Jun. 5, 2009; patentapplication No. 12/479,377, entitled “Method and System for InitializingStorage in a Storage System” by Galloway et al., filed Jun. 5, 2009;patent application No. 12/479,434, entitled “Method and System forRebuilding Data in a Distributed RAID System” by Galloway et al., filedJun. 5, 2009; and patent application No. 12/479,394, entitled “Methodand System for Placement of Data on a Storage Device” by Galloway etal., filed Jun. 5, 2009 all of which are incorporated fully herein byreference.

A brief discussion of context particularly with respect to data storagemay now be helpful. As discussed above, RAID systems divide andreplicate data across multiple hard disk drives (or other types ofstorage media), collectively referred to as an array, to increasereliability and in some cases improve throughput of computing devices(known as a host) using these RAID systems for storage. However, currentimplementations of RAID may have a variety of problems. These problemsmay stem from limitations imposed by the architecture of these RAIDsystems, from the configuration or layout of the data on the diskscomprising a RAID system or from the need to use custom hardware orsoftware to implement these solutions, raising the costs associated withimplementing such a solution. It is desired to substantially amelioratethese problems, among others.

To that end, attention is now directed to the systems and methods of thepresent invention. Embodiments of these systems and methods provide adistributed RAID system comprising a set of data banks. Moreparticularly, in certain embodiments of a distributed RAID system eachdata bank has a set of associated storage media and executes a similardistributed RAID application. The distributed RAID applications on eachof the data banks coordinate among themselves to distribute and controldata flow associated with implementing a level of RAID in conjunctionwith data stored on the associated storage media of the data banks.

Specifically, in certain embodiments, a volume with an associated RAIDlevel may be created using the distributed RAID system. Each of thedistributed RAID applications can then coordinate operations associatedwith data of that volume such that data associated with that volume orthe implementation of the desired RAID level in conjunction with thatvolume may be stored on the multiple data banks of the distributed RAIDsystem.

By coordinating the implementation of a level of RAID in conjunctionwith a volume by storing both data of the volume and data associatedwith the implementation of RAID on multiple data banks using similardistributed RAID applications executing on each of those data banks anumber of advantages may be achieved. Namely, different storage volumesmay be allotted, with one or more of the volumes implemented inconjunction with different RAID levels. Moreover, as the coordination ofstorage and the implementation of RAID across the data banks isaccomplished using substantially identical distributed RAIDapplications, in many cases standard or off-the-shelf hardware, such asstandard x86 based servers and storage media may be utilized. Many otheradvantages may also be realized utilizing embodiments presented hereinor other embodiments, and such advantages, which may or may not bepointed out in particular detail, will be realized after reading thisdisclosure.

Turning now to FIG. 1, a block diagram of an architecture for a systemwhich utilizes one embodiment of a distributed RAID system is depicted.Distributed RAID system 100 includes a set of data banks 110, each databank 110 communicatively coupled to both of switches 120. Each ofswitches 120 is also communicatively coupled to each host 102, such thata host 102 may communicate with each data bank 110 through a set ofpaths corresponding to a particular data bank 110, each path comprisingone of the switches 120.

The communicative coupling between data banks 110, switches 120 andhosts 102 may be accomplished using almost any transport medium (eitherwired or wireless) desired, including Ethernet, SCSI, iSCSI, FibreChannel, serial attached SCSI (“SAS”), advanced technology attachment(“ATA”), serial ATA (“SATA”) or other protocols known in the art.Furthermore, the communicative coupling may be implemented inconjunction with a communications network such as the Internet, a LAN, aWAN, a wireless network or any other communications network known in theart.

In one embodiment, then, using a commands protocol, such as iSCSI, SCSI,etc., hosts 102 may communicate with data banks 110 to manipulate data.More particularly, each of data banks 110 comprises storage media (aswill be explained in more detail later on herein). Collectively, thestorage media in data banks 110 may be virtualized and presented tohosts 102 as one or more contiguous blocks of storage, storage devices,etc. For example, when the iSCSI protocol is utilized the storage mediain data banks 110 may be presented to hosts 102 as a SCSI target with,in one embodiment, multiple ports.

Thus, during operation, in one embodiment a host 102 (or a user at ahost 102 or interfacing with data bank 110) may request the creation ofa volume and specify a level of RAID to be implemented in conjunctionwith that volume. Data associated with that volume and theimplementation of the desired level RAID in association with that volumeis stored across data banks 110. The hosts 102 may then access thisvolume using logical address corresponding to the volume or a portionthereof. In this manner, hosts 102 can utilize created volumes ofstorage and fault tolerance can be achieved in conjunction with thesevolumes substantially invisibly to hosts 102.

The virtualization of storage and the implementation of RAID utilizingdata banks 110 may be better understood with reference to FIG. 2A whichdepicts a block diagram of one embodiment of a data bank 110 computeroperable to implement distributed RAID. Here, data bank 110 comprises adata store 250, and a processor 202 operable to execute instructionsstored on a computer readable medium, where the instructions areoperable to implement distributed RAID application 210. Distributed RAIDapplication 210 may periodically issue heartbeat communications todistributed RAID applications 210 on other data banks 110 to determineif there has been a fault with respect to that data bank 110. If thedistributed RAID application 210 determines that another data bank 110is experiencing a fault it may set one or more fault flags correspondingto that data bank 110. Using these fault flags for each distributed RAIDapplication 210 on each data bank 110 a particular distributed RAIDapplication 210 may determine if a certain data bank 110 is faulty.

Distributed RAID application 210 may also have access (for example, toread, write, issue commands, etc.) to data store 250 comprising one ormore storage media, which may for example be disks 252 operatingaccording to almost any protocol known, such as SATA, PATA, FC, etc.where each of the disks 252 may, or may not, be of equal size.Distributed RAID application 210, executing on each of data banks 110can allow the allocation of and use of volumes using data stores 250across data banks 110 and the implementation of RAID in conjunction withthese volumes utilizing a set of global tables 240 shared between databanks 110, a set of local tables 245 and write cache 260, all of whichmay be stored in a memory 230 (which may be data store 250 or anothermemory altogether).

FIG. 2B depicts a block diagram of one embodiment of a hardwarearchitecture which may be used to implement data bank 110 computeroperable to implement distributed RAID. In this architectural example,data bank 110 comprises one or more processors 202 which may adhere tothe Intel x86 architecture or some other architecture altogether and amemory 230 coupled through a bus to I/O controller hub 212, which in oneembodiment may be a southbridge chip or the like. The I/O controller hub212 may, in turn, be coupled to and control a bus 272 such as a PCI-Xbus, PCI-express bus, etc. Coupled to this bus 272 are one or more diskcontrollers 262 such as, for example, an LSI 1068 SATA/SAS controller.Each of these disk controllers 262 is coupled to one or more disks 252,where collectively these disks 252 may comprise data store 250.Additionally, one or more network interfaces 282 may also be coupled tobus 272. These network interfaces 282 may be network interfaces (such asEthernet, etc.) which are included on motherboards, may comprise one ormore network interface cards configured to interface via one or moreprotocols such as Ethernet, fibre channel, etc. or may be some othertype of network interface such that data bank 110 may communicate withswitched 120 through these network interfaces 282.

Moving now to FIG. 3, one embodiment of a method for the allocation ofvolumes and the laying out of data associated with these volumes in datastores 250 across data banks 110 is illustrated. Initially, beforevolumes are to be allocated on a disk 252 of data store 250, the disk252 may be formatted at step 305. As discussed above, in order to havethe ability to easy and simply recover from any failures redundancy datamay need to be accurate relative to any corresponding stored data. Inmany cases, this may entail that disks 252 utilized to store a volume beformatted by calculating redundancy data from the current data in theareas on disk 252 where portions of a volume are to be stored, eventhough data stored at these areas may currently be garbage values. Thesecalculations may consume an undesirably large amount of time.

Furthermore, in a distributed RAID environment such as that detailedwith respect to FIG. 1, other problems may present themselves. Morespecifically, as different portions of a volume may be stored ondifferent data banks 110 and redundancy data corresponding to the volumemay also be stored on various data banks 110, accomplishing this type offormatting may additionally require a great deal of communicationbetween distributed RAID applications 210 on data banks 110, consumingprocessor cycles and communication bandwidth.

Thus, in one embodiment, to ensure that redundancy data corresponding toan area of a disk where data of a volume is to be stored is accuraterelative to the area of disk 252 where that data of the volume is to bestored, a zero value may be written to the areas on disks 252 where datacorresponding to the volume is to be stored and the areas on disk 252where redundancy data is to be stored. By zeroing out both the areas ofa disk 252 where data of a volume is to be stored and areas of disks 252where redundancy data is to be stored it can be guaranteed that any dataof the volume can be recreated from its corresponding redundancy data.

Zeroing disks 252 may have other advantages. Namely that no complexcalculations may need to be performed to determine redundancy data andno communications between distributed RAID applications 210 may be toachieve relative accuracy between areas where a volume is to be storedand redundancy data corresponding to those areas.

Importantly, by zeroing out areas of disks 252 for use with a volume andits corresponding redundancy data a significant delay in the usabilityof RAID system 100 may be avoided. These advantages may be attainedthrough the use of a process which substantially continuously duringoperation zeros out unallocated areas of disks 252 resulting, forexample, from the initial use of distributed RAID system 100, theinstallation of new disks 252, the deletion of a volume, etc. In theseinstances, currently unallocated (i.e. not currently allocated) areas ofdisks 252 on each of data banks 110 may have zeros written to them(referred to as “zeroing” the area).

The unallocated areas of disks 252 which have been zeroed may be trackedsuch that when a command corresponding to a portion of a volume orredundancy data associated with a portion of a volume is received at adata bank 110 to which that portion is assigned, distributed RAIDapplication 210 may check to determine if that portion has been assigneda corresponding area of disks 252 on data bank 110 where that portionhas been assigned. If no corresponding area of disks 252 on data bank110 has been assigned, distributed RAID application 210 may select anarea of disks 252 which has been zeroed and assign this area of disks252 to the portion of the volume or corresponding redundancy data.

By simultaneously zeroing out any unassigned areas which have notpreviously been zeroed and waiting until a command corresponding to aportion of a volume or redundancy data is received to assign a zeroedarea of disks 252 to that portion distributed RAID system 100 mayoperate substantially immediately without a long involved formattingprocess and new disks 252 may be added and volumes deleted or freedrelatively unobtrusively to the operation of distributed RAID system100.

It will be noted, therefore, after reading the above that step 305 inwhich the disks 252 are formatted may be accomplished before, during orafter the creation of a volume with respect to distributed RAID system100 and that the placement of step 305 (and all other steps in all theflow diagrams herein) implies no order to the steps. As will also benoted after a thorough review of the rest of the steps in FIG. 3 and theremainder of the disclosure, a volume may be created and portions of thevolume and redundancy data corresponding to the volume assigned to adata bank 110 before physical areas on disks 252 on these data banks 110have been assigned to store the portions of the volume or redundancydata and that, furthermore, the zeroing of the physical areas on disks252 used to store the data corresponding to such portions may occurbefore the creation of the volume or after the creation of the volumebut before these physical areas are assigned to corresponding portionsof the volume or redundancy data (as discussed in more detail later).

These locations may be better explained with reference to the concept ofa segment which may be utilized by embodiments of a distributed RAIDapplication 210, where a segment may be the size of 2048 logical blockaddresses (LBAs) (or some other size) and the size of the logical blockaddress corresponds to the sector size of a disk 252. Disks 252 in thedata store 250 on each of data banks 110 may therefore be separated intoequal size segments (for example, 1 MB) at step 310. These segments maycorrespond to one or more contiguous data blocks of a disk drive 252.Therefore, when a user or host 102 requests the creation of a volumefrom distributed RAID application 210 at step 320 and specifies a levelof RAID which will be used in conjunction with that volume at step 330,a number of these segments corresponding to the requested size of thevolume plus the number of segments desired to implement the desiredlevel of RAID in conjunction with the volume may be assigned to thevolume at step 340.

Thus, the volume comprises a number of segments (also referred to aslogical segments), where each of these segments may be associated with aparticular data bank 110 such that the data bank 110 may be assigned tomanage that segment of the volume. This segment may, for example, may bethe size of 2048 logical block addresses (LBAs), where the size of thelogical block address corresponds to the size sector size of a disk 252(other arrangements and sizes will also be possible). In most cases thephysical storage comprising that segment of the volume will be stored inthe data store 250 of the data bank 110 which manages that segment,however, in other cases the data corresponding to that segment may bestored in the data store 205 of a different data bank 110 (in otherwords, in certain cases the data bank 110 comprising the distributedRAID application which manages that segment may be distinct from thedata bank 110 comprising the data store 250 which stores the datacorresponding to that segment).

In one embodiment, the allocation of segments to data banks 110corresponding to a particular volume may be accomplished by determininga random permutation of the set of data banks 110 corresponding to thedistributed RAID system 100. Thus, if there are six data banks a randompermutation of size six, where the random permutation comprises each ofthe data banks may be determined such that the segments may assigned toeach of the data banks consecutively in the order of the randompermutation.

For example, suppose there are four data banks 110 in a distributed RAIDsystem (call them data bank1, data bank2, etc.). A random permutation ofdata bank2, data bank4, data bank1 and data bank3 may be determined. Inthis case, the first segment corresponding to a volume is on data bank2,the second segment may be on data bank4, the third on data bank1, thefourth on data bank 3 and the fifth back again on data bank 4. In thisway, the location of a particular segment corresponding with the volumemay be determined mathematically if the random permutation correspondingto the volume is known.

As mentioned the user may specify that a level of RAID is to beimplemented in conjunction with a volume at step 330. In this case,distributed RAID application 210 may ensure that any data correspondingto the implementation of RAID in conjunction with a volume is stored atan appropriate location at step 350 such that the RAID information isappropriately distributed across data banks 110 to ensure that thedesired level of RAID is achieved.

For example, if it is desired to implement RAID 5 in conjunction with avolume, distributed RAID application 210 may determine a desired RAIDparity group size (for example, based on a user configured RAID set orotherwise determined). This determination may be based on the number ofdata banks 110 in the distributed RAID system and may, in oneembodiment, be one less than the number of data banks 110 (plus anadditional one to account for the parity data).

To illustrate, if there were five data banks 110, for every foursegments which store data associated with the volume (referred to asdata segments), one segment would be dedicated to parity and the parityfor the four segments calculated and stored in this parity segment,where the parity segment would be dedicated in a data bank 110 whosedata store 250 does not comprise the data segments from which the paritydata of the parity segment was calculated.

At this point, each segment corresponding to a logical volume has beenassigned to a particular data bank 110 and any segments 100 to beutilized to store RAID data corresponding to the volume (referred toherein interchangeably as redundancy segments or parity segments,without loss of general applicability to the use of the segment to storeany type of redundancy data associated with the implementation of anylevel of RAID in conjunction with a volume) have also been assigned to adata bank 110, however, physical sectors of the disks 252 of the datastores 250 of the data banks may not have yet been assigned to store thedata corresponding to those segments. Thus, at step 360 physicalsegments of disks 252 on the data bank 110 to which a logical segment ofthe volume has been assigned may be determined and assigned to thelogical segments. This segment mapping may be stored in the local tables245 of each data bank 110. This assignment may, as mentioned earlier,take place at some later point, for example, when a command firstattempts to write a logical segment.

When making this assignment, in one embodiment the areas differentperformance characteristics of disks 252 may be accounted for relativeto the accessed logical segment. In other words, disks 252 may havesegments which are more efficiently accessed than other segments of thesame disk. Therefore, in one embodiment it may desirable to assignphysical segments of a disk 252 based upon criteria associated with thelogical segment. The characteristics may include for example, suchthings as a quality of service designation associated with a volumecorresponding to the logical segment, a number of accesses to the volumecomprising the logical segment, etc.

At step 370, then, information corresponding to the volume may bestored, such that the location of segments corresponding to the volume,or segment corresponding to the implementation of RAID in conjunctionwith the volume, may be determined from this stored information. Thisstored information (collectively referred to as mapping data) maytherefore include an identification for the volume, the randompermutation corresponding to the volume (for example, indicating theorder of data banks 110 on which the segments are located) and theparity group size of any RAID implementation (for example, if the volumecorresponds to a 4+1 RAID set, a 7+1 RAID set, if RAID 1 is implemented,etc.). This data may be stored, for example, in global tables 240 suchthat it can be communicated to other distributed RAID applications 210on other data banks 110 to ensure that at least a portion of the set oftables 240 associated with each distributed RAID application 210 remainssubstantially consistent and the location.

The above description may be better understood with reference to FIG. 4which depicts one example of one embodiment of a distributed RAID systemwith five data banks 110. Here, each data store 250 of each data bank110 has been laid out as a set of equally sized segments 402, which forpurposes of this example will be assumed to be 1 MB in size. Suppose,now that a host 102 requests a volume of 8 MB with RAID level 5 fromdistributed RAID application 210 b on data bank 110 b. In this case,distributed RAID application 210 b may determine that eight segments 402are required for the data portion of the volume while two segments 402segments may be required to store the redundancy data for the volume inconjunction with implementation of the desired RAID level for thevolume. Distributed RAID application 210 b may then determine a randompermutation for the volume. For purposes of this example, assume thatthe random permutation is: data bank 110 b, data bank 110 d, data bank110 a, data bank 110 c, and data bank 110 e. Thus, data bank 110 b maybe assigned segment 402 a, the first segment of the requested volume,data bank 110 d may be assigned segment 402 b, the second segment of therequested volume, data bank 110 a may be assigned segment 402 c, thethird segment of the requested volume, data bank 110 c may be assignedsegment 402 d, the fourth segment of the requested volume and data bank110 e may be assigned segment 402 e, the fifth segment of the requestedvolume. The assignment then begins again with the first data bank 110 ofthe random order such that data bank 110 b may be assigned segment 402f, the sixth segment of the requested volume, data bank 110 d may beassigned segment 402 g, the sixth segment of the requested volume anddata bank 110 a may be assigned segment 402 h, the eighth segment of therequested volume.

Once the data segments 402 a-402 h for storing data associated with thevolume have been assigned, distributed RAID application 210 b may assignsegments 402 for any data associated with the implementation of thatRAID level. In this example, as RAID 5 is desired with respect to thevolume, distributed RAID application 210 b may determine that as fivedata banks 110 are being utilized a (4+1) parity set may be desired.Distributed RAID application 210 b may then determine that to store theparity to implement RAID 5 in conjunction with eight segments 402 anadditional two segments 402 may be needed.

Furthermore, it may be desired that the parity created utilizing aparticular set of data segments 402 will not be stored on a data bank110 having any of those set of data segments 402 in its data store.Thus, distributed RAID application 210 b may also determine a locationwhere each of the parity segments will be allocated based on thedetermined RAID parity group size, the location of the first datasegment 402 a, etc. Here, parity segment 402 i which will store theparity data corresponding to the data stored in data segments 402 a, 402b, 402 c and 402 d will be allocated in data store 250 c of data bank110 c while parity segment 402 j which will store the parity datacorresponding to the data stored in data segments 402 e, 402 f, 402 gand 402 h will be allocated in data store 250 e of data bank 110 e.Notice here that the parity segments 402 i, 402 j which will store theparity information associated with the implementation of RAID inconjunction with the volume comprising data segments 402 a-402 h arelaid out and sized substantially identically to as those segments 402a-402 h which store the data associated with the volume.

Thus, when a host 102 accesses the volume, a request with a logicaladdress corresponding to the first data segment of the volume maycorrespond to data segment 402 a on data bank 110 b, a request with alogical address corresponding to the second data segment of the volumemay correspond to data segment 402 b on data bank 110 d, etc. Noticehere, that the allocated data segments 402 a-402 h may reside ondifferent data banks 110 and that the location of any allocated datasegment 402 a-402 h may be determined using the random permutationassociated with that volume (for example, as stored in global tables 240at data banks 110). As discussed above, however, data stores 250 on databanks 110 have been virtualized, thus the requesting host may not beaware of the location of the data segments 402 in data stores 250, thatmultiple data stores 250 exist, that data stores 250 are spread acrossmultiple data banks 110, etc. Host 102 believes it is addressing asingle contiguous volume.

It will be apparent that the location of the data segments 402 on databanks 110 (and the corresponding random permutation of data banks 110)in this example is for purposes of illustration and that the datasegments 402 of a volume may be located on any of data stores 250 on anyof the data banks 110 according to almost any random, or other,permutation. Furthermore, it will be noted that while each of segments402 is in this example 1 MB, these may be of any size without loss ofgenerality and that a 1 MB size has been chosen solely for ease ofillustration.

As can be seen from the above description then, the location of aparticular data segment 402 or parity segment 402 can be determinedalgorithmically (for example, using the same random permutation used toassign segments for the volume, locate the parity segments for thevolume, etc.) using the random permutation associated with the volumeand the RAID parity group size. Thus, the information may be stored inconjunction with an identification corresponding to the volume, forexample in set of global tables 240. Furthermore, these global tables240 may be communicated between data banks 110, or otherwise updated,such that at least portions of the set of global tables 240 in each ofthe data banks 110 may be kept substantially consistent.

It may be helpful here to briefly delve into more detail regardingglobal tables 240 associated with distributed RAID application 210. Asdiscussed, in one embodiment, global tables 240 may store informationassociated with volumes created by distributed RAID application 210where those tables 240 can be used to determine a data bank 110associated with a data segment within that volume or where a paritysegment associated with a data segment corresponding to that volume islocated. Global tables 240 may therefore comprise a set of tables, eachtable corresponding to a volume implemented with respect to databanks110. In particular, one of these tables 240 may contain data which maybe used to identify a data bank 110 whose data store 250 comprises acertain segment of a volume. Specifically, this table may be used tocorrelate a logical address associated with a volume with the data bank110 where the segment (data, redundancy, etc.) corresponding to thatlogical address is stored.

FIG. 5 depicts a graphical representation of one embodiment of this typeof table, where each volume may have an associated instance of such atable associated. Table 550 includes entries for LV number 504, segmentsize 508, segment count 512, quality of service (QOS) 514, range count518, information for range entries, including in the embodiment depicteda first range 524 a and a second range 524 b and any additional rangeentries 524 n.

LV number 504 is a unique number used to identify a particular volume,segment size 508 corresponds to the size of the segments used toimplement the volume, segment count 512 corresponds to the number ofsegments corresponding to the logical volume (for example, both thenumber of data segments and redundancy segments, just the number of datasegments, etc), QOS 514 indicates the quality of service which it isdesired to implement with respect to the volume (note that this QOSindicator may indicate a priority to be given to that volume relative toother volumes stored on data banks 110) and range count 518 indicates anumber of ranges associated with the volume, while range entries 524each correspond to one of those ranges.

A range may correspond to a particular data bank 110 order and RAIDimplementation. Multiple ranges may be utilized to implement a volumefor a variety of reasons. Specifically, for example, multiple ranges maybe utilized in conjunction with a volume because different data stores250 at different data banks 110 may have different amounts of storage indata store 250 available for use. This may lead to a situation where forexample, for a first range of a volume all data banks 110 may beutilized in conjunction with a first RAID implementation while in asecond range of a volume fewer than all the data banks 110 available maybe utilized in conjunction with a second RAID implementation (where thefirst and second RAID implementations may, in fact, be different levelsthan one another). Each of these ranges may therefore correspond tosegments laid out according to different data bank 110 orders (forexample, random permutations, etc.), having a different number of databanks 110 available for use, a different type of RAID, etc.

To illustrate using a concrete example, brief reference is made back toFIG. 4. Suppose that the volume of 8 MB with RAID level 5 is laid out asshown, where the data segments are laid out according to the order databank 110 b, data bank 110 d, data bank 110 a, data bank 110 c, and databank 110 e and RAID 5 is implemented in conjunction with the volumeutilizing a (4+1) parity set may be desired with the parity segmentsassigned in data store 250 c of data bank 110 c and data store 250 e ofdata bank 110 e.

Now suppose that it is requested to add an additional 3 MB to thisvolume. However, suppose in this instance that data stores 250 of databanks 110 e, 110 c and 110 d have no more room. Thus, in this case theonly solution may be to allocate the additional desired 3 MB betweendata banks 110 a and 110 b which have remaining storage in data stores250. Furthermore, as only two data banks 110 may be available for use itmay only be possible to utilize a RAID level of 1 instead of RAID 5 asutilized with the first 8 MB of the volume. Thus, in this case the first8 MB of the volume may correspond to a first range, and have a firstrange entry in a table corresponding to the volume with a first set ofvalues while the next 3 MB of the volume may correspond to a secondrange, and have a second range entry in a table corresponding to thevolume with a second set of values. As may be apparent after readingthis disclosure, this type of occurrence may occur with some frequency.

Returning to FIG. 5, to deal with these types of situations, amongothers, each range of a volume may have an entry in a table 550 suchthat the location of segments in that particular range may be determinedfrom the range entry corresponding to that range. Entries 524 for eachof the ranges of the volume corresponding to the table 550 areassociated with range count 518. In one embodiment, range count 518 maycorrespond to the number of ranges of a volume such that the number ofrange entries 524 corresponds to the range count 518. While only rangeentries 524 a and 524 b are shown it will be noted that the number ofrange entries 524 in a table will depend on the number of rangescorresponding to the volume to which that table corresponds. Thus, if avolume is divided into three ranges, there will be three range entries524 in table 550 such that there is a range entry 524 comprisinginformation for each range of the volume corresponding to table 550.

Information for a range entry 524 includes type 526, start 530, end 534,network RAID 538, network RAID size 542, disk RAID 546, disk RAID size550, databank count 554, databank order 558 and a disk count 562 anddisk order 566 corresponding to each data bank 110 used to storesegments associated with range 524 (in other words there will be a diskcount 562 and disk order 566 equal to databank count 554 of that rangeentry 524). Type 526 describes the type of the range corresponding toinformation for range entry 524: for example, normal, source (SRC),destination (DST) or other type of range. Start 230 is the first logicalsegment address of the range of the volume corresponding to range entry524. End 234 is the last logical segment address of the rangecorresponding to information for the range of the volume correspondingto range entry 524. Other arrangements are also possible, for example,end 524 may be a count which is the maximum number of segments or blocksin the range, etc.

Databank count 554 may correspond to the number of data banks 110 onwhich the range corresponding to the range entry resides, databank order558 may be the order in which segments in that range were assigned todata banks 110 while network RAID 538, network RAID size 542, disk RAID546 and disk RAID size 552 may correspond to the type of RAIDimplemented in conjunction with the range of the volume corresponding torange entry 524.

Network RAID 538 is the type of RAID being implemented in associationwith the volume corresponding to the table 550, for example, RAID 0,RAID 1 or RAID 5 or other RAID types. Network RAID Size 542 is theparity group size of the RAID type used in the range. The Network RAIDSize 542 may be limited by the number of data banks 110 in the range tobe less than or equal to the number of databanks in the rangecorresponding to information for range 524. Disk RAID 546 is the type ofRAID being implemented across disks in the databanks in the range. DiskRAID size 552 may be the parity group size of the RAID type used acrossthe disks 252 in the data store 250 of each data bank 110 and may belimited to be less than or equal to the number of disks in the databank.In embodiments, RAID across the disks in the databanks 110 in the rangeis optional and may or may not be used. In such embodiments, either DiskRAID 546, Disk RAID Size 552 or both may not be used or may be omitted.

Data bank count 554 is the number of databanks in the range and Databankorder 558 is the order in which RAID is implemented (for example,striped) across the data banks 110 in the range. For example, data banks110 may have data corresponding to the logical addresses of the volumesaved in a certain order and databank order 558 corresponds to thisorder. Disk count 562 is the number of disks within a data bank 110 ofthe range and disk order 566 is the order in which RAID is implementedacross disks of a particular databank 110. For example, disks 252 mayhave segments saved to them in a certain order and disk order 566 is theorder in which segments are stored across disks 252 in a data bank 110.Thus, for each databank 110 used to store segments of the rangeassociated with the range entry 524 there will be a corresponding diskcount 562 and disk order 566 (in other words the number of disk counts562 and disk orders 566 will, in one embodiment, be equal to databankcount 554 of that range entry 524). In embodiments, RAID across disks252 in the data banks 110 is optional and may not be used. It will benoted that while table 550 has been described with specificity, thisdescription is by way of example, not limitation and other forms oftable 550 may be utilized. For example, a virtual table may be usedinstead of table 550 and may explicitly list the segment 402 and databank 110 corresponding to each logical address.

Thus, as discussed earlier, information in table 550 may be used toidentify a data bank 110 comprising a data segment 402 corresponding toa logical address (referenced by a host 102 in a command or in any othercontext). For example, knowing the size of segments 402 and using start530, end 534, the range entry 524 corresponding to the address, etc.,the particular data bank 110 corresponding to a logical address of thevolume can be determined.

While one or more portions of tables 240 may be substantially identicalacross all data banks 110 and may describe one or more logical volumeswhich span one or more data banks 110 as described above, other tables245 on a data bank 110 may be distinct to the data bank 110 to which itcorresponds (for instance, table 245 may be unique to the data bank 110on which the corresponding distributed RAID application 210 isexecuting). This table 245 may comprise data pertaining to each disk 252contained in the data store 250 of the corresponding data bank 110 andmay comprise information on where information is stored on or amongdisks 252 of the data store, for example, the sector of a disk 252 wherea segment 402 assigned to the data bank 110 is located in data store250.

In FIG. 6 a graphical representation of one embodiment of this type oftable is depicted. Table 660 may be stored at a particular data bank 110and comprise multiple disk tables 670, each of the disk tables 670corresponding to a disk 252 of the data store 250 within that data bank110 and listing the location of the segments stored within that disk252. More specifically, in most cases disks 252 are divided intophysical sectors, each physical sector having a corresponding address orrange of addresses.

A disk table 670 may be a mapping table which can be utilized todetermine the location of a sector of a disk 252 of the data bank 110where a segment of a volume is stored. Thus, using a table 670 theaddress of a sector on a disk 252 corresponding to a segment of a volumecan be determined. Furthermore, the table may contain one or more flagsor descriptive bits per entry corresponding to a segment or sector ofthe disk, describing the sector or segment stored at that sector.

Referring now to FIG. 7, a graphical representation of one embodiment ofa disk table 670 is depicted. Disk table 670 has multiple entries, eachentry corresponding to a physical segment of the corresponding disk suchthat the entries of disk table 670 describe the physical segments of thedisk 252. Each entry in disk table 670 may also include one or moreflags or bit fields describing the physical segment or segment of thevolume stored at the corresponding sector. More particularly, as shownin FIG. 7, in one embodiment entries in disk table 670 include fieldsfor a logical volume (LV) number, logical segment number, address spaceand sector state. LV number identifies the logical volume to which datastored at that physical segment corresponds. Logical segment numberidentifies the segment of the logical volume corresponding to that data.Address space identifies the segment stored as ‘data’ or ‘redundancy’. Avalue of ‘data’ may indicates that data is stored at the sectorrepresented by the entry, whereas a value of ‘redundancy’ indicates thatthe information stored at the sector may be used for RAID dataprotection and, depending upon the RAID level, may be redundant data,mirrored data or parity information. Sector state indicates the state ofthe segment as being ‘allocated’, ‘zeroed’ or ‘dirty’. ‘Allocated’indicates the segment has been allocated and may comprise valid data.‘Zeroed’ indicates the segment has been zeroed out by writing zeros tothe segment and ‘dirty’ indicates the segment may comprise garbage areotherwise unusable or undesirable values, for example because thesegment has not been zeroed out or allocated, may be storing random bitsor data. In one embodiment, for example, for a new disk all segments ofthe disk may be marked as dirty in a disk table corresponding to the newor newly added disk.

After reading the above description of the tables it will be apparentthat distributed RAID application 210 may utilize the global tables 240to determine which segment corresponds to a logical address of a volume,on which data bank 110 segments corresponding to a volume (either dataor redundancy segments) are located, which segment of a volumecorresponds to a logical address of a volume, where RAID data (paritydata, mirror data, other types of redundancy data, etc.) associated witha segment of a volume is located, which disk 252 on a particulardatabank 110 comprises a segment or other information regarding volumes,segments, or disks 252 corresponding to that particular data bank 110 orother information regarding volumes, segments 402, data banks 110, RAIDdata, etc.

Similarly, distributed RAID application 210 on each individual data bank110 may use local tables 245 on that data bank 110 to determine where onthat data bank 110 (which sector(s) of disk 252, etc.) a particularsegment is located or other information regarding volumes, segments, ordisks 252 corresponding to that particular data bank 110.

Using the combination of the global table 240 shared between data banks110 and the local tables 245 corresponding to each individual data bank110 then, certain operations may be performed by the distributed RAIDapplications 210 on data banks 110 in cooperation with one another.These types of operations will now be discussed in more detail.Specifically, one embodiment of the implementation of a READ command anda WRITE command on a volume where RAID level 5 has been implemented inconjunction with the volume will now be discussed in more detailfollowed by concrete examples of the implementation of these commandswith respect to an example distributed RAID system. It will be noted howother types of embodiments, commands, RAID levels, etc. may beimplemented after a thorough review of this disclosure.

Looking first at FIG. 8, a flow diagram for one embodiment of a methodfor implementing a READ command in a distributed RAID system isdepicted. This READ command may be sent by a host 102 to a data bank 110through a switch 120 or from one data bank 110 to another data bank 110.In certain embodiments, host 102 may comprise one or more applicationsand associated routing information such that a READ command may berouted from the host 102 issuing the command to an appropriate data bank110 along a path between the issuing host 102 and the appropriate databank 110. In other cases, however, no such application or routinginformation may be present on host 102 and thus a READ command issuedfrom a host 102 may be routed to any of data banks 110. It is the lattercase that will be illustrated in this embodiment. After reviewing thedescription of this embodiment, however, it will be noted by those ofskill in the art which steps are applicable to the former case as well.

At step 810, then, a READ command may be received at a data bank 110.The distributed RAID application 210 on data bank 110 may determine, atstep 820, a segment of a volume which corresponds to a logical addressreferenced in the received READ command and on which data bank 110 thesegment of the volume is stored at step 830. As discussed above, thisinformation may be determined using the global tables 240 associatedwith the distributed RAID application 210. If the data bank 110 which isstoring the segment is the same as the data bank 110 which received theREAD command (as determined at step 832) the requested data can beobtained from the appropriate disk 252 of the data store 250 on thereceiving data bank 110 at step 840 and at step 850 the READ commandresponded to. As discussed above, the particular disk 252 of a datastore 250 of the data bank 110 on which a segment is stored can bedetermined using global tables 240 while the location on that disk 252where the data corresponding to the segment is stored may be determinedusing local tables 245 which may be used to map a segment of a volume toa physical location on a disk 252. If the receiving data bank 110received the READ command from the host 102 the host 102 may beresponded to while if the receiving data bank 110 received the READcommand from another data bank 110 the response may be sent to thedistributed RAID application 210 on the data bank 110 which issued theREAD command.

If, however, the segment is stored on a remote data bank 110 (a databank 110 other than the one which received the command) at step 860 theREAD command may be sent to the distributed RAID application 210 at theremote data bank 110. In one embodiment, this READ command may becommunicated to the distributed RAID application 210 at the remote databank 110 using a command format utilized by distributed RAID application210. This command, while providing pertinent information of the originalREAD command may also instruct the distributed RAID application toreturn the result of the READ command to the data bank 110 whichoriginally received that READ command, or to perform otherfunctionality. Accordingly, after the READ command is sent to the remotedata bank 110 at step 870 a response comprising the requested data maybe received from the remote data bank 110 and at step 880 the receivedREAD command responded to using the data received in that response.

Moving, now to FIGS. 9A and 9B, a flow diagram for one embodiment of amethod for implementing a WRITE command in a distributed RAID system isdepicted. This WRITE command may be sent by a host 102 to a data bank110 through a switch 120 or from one data bank 110 to another data bank110. In certain embodiments, host 102 may comprise one or moreapplications and associated routing information such that a WRITEcommand may be routed from the host 102 issuing the command to anappropriate data bank 110 along a path between the issuing host 102 andan appropriate data bank 110. In other cases, however, no suchapplication or routing information may be present on host 102 and thus aWRITE command issued from a host 102 may be routed to any of data banks110. It is the latter case that will be illustrated in this embodiment.After reviewing the description of this embodiment, however, it will benoted by those of skill in the art which steps are applicable to theformer case as well.

At step 910, then, a WRITE command may be received at a receiving databank 110. The distributed RAID application 210 on receiving data bank110 may then determine at steps 920, 930 and 940 the segment of thevolume corresponding to a logical address referenced by the WRITEcommand, the location of that segment (for example, which data banks 110is storing the data corresponding to that segment) and the location ofthe parity corresponding to that segment (for example, which data bank110 is storing the segment where parity data created from the datacorresponding to that segment is stored). As discussed above, thelocation of both the data segment and the parity segment may bedetermined using global tables 240 stored on the receiving data bank110.

If neither the data segment (the segment storing the data) nor theredundancy segment (in other words, where the parity or other type ofredundancy data created from the data segment) is stored on thereceiving data bank 110 (as determined at steps 950 and 960) the WRITEcommand may be communicated to the distributed RAID application 210 onthe remote data bank 110 on which the data segment is stored at step 964and to the distributed RAID application 210 on the remote parity databank 110 on which the parity segment is stored at step 966. In oneembodiment, this WRITE command may be communicated to the distributedRAID applications 210 at the remote data bank 110 and the remote paritydata bank 110 using a command format utilized by distributed RAIDapplications 210. This command, while providing pertinent information ofthe original WRITE command may also instruct a distributed RAIDapplication 210 to perform other desired functionality.

Accordingly, after the WRITE command is sent to the remote data bank 110and the remote parity data bank completion notifications may be receivedfrom the distributed RAID applications 210 on the remote data bank 110and the remote parity data bank 110 at steps 968 and 970. Once theseacknowledgments are received the WRITE command may be responded to bythe distributed RAID application 210 on the receiving data bank 110.

Returning to step 950, if, however, the data segment is stored at thereceiving data bank 110, it may be determined if the WRITE command wasreceived from a host 102 or another data bank 110 at step 952. If theWRITE command was received from a host 102 the WRITE command may becommunicated to the distributed RAID application 210 on the remoteparity data bank 110 at step 976 and placed in the write cache of thereceiving data bank 110 at step 974. After receiving a completionnotification from the distributed RAID applications 210 on the remoteparity data bank 110 at step 978, the WRITE command may be responded toby the distributed RAID application 210 on the receiving data bank 110at step 980 (for example, a response sent to the host 102). Furthermore,the WRITE command itself may be processed at step 982. This process mayentail the storing of data associated with the WRITE command to the datasegment stored on the receiving data bank 110 or other functionality.

On the other hand, if the WRITE command was not received from a host atstep 952 this may indicate that the WRITE command was received fromanother data bank 110 (which, in many cases, may have been the data bank110 which originally received the WRITE command from a host 102). Inthis case, the data bank 110 may place the received WRITE command in itswrite cache at step 984 and sends a completion notification to theissuing data bank 110 at step 986. At some later point then, the WRITEcommand itself may be processed at step 988.

Returning again to step 950, if the data segment is not stored at thereceiving data bank 110 but the parity segment is stored at thereceiving data bank 110, as determined at step 960, it may be determinedif the WRITE command was received from a host 102 or another data bank110 at step 962. If the WRITE command was received from a host 102 theWRITE command may be communicated to the distributed RAID application210 on the remote data bank 110 where the data segment corresponding tothe WRITE is stored at step 1002 and placed in the write cache of thereceiving data bank 110 at step 1000. After receiving a completionnotification from the distributed RAID applications 210 on the remotedata bank 110 at step 1004 the WRITE command may be responded to by thedistributed RAID application 210 on the receiving data bank 110 at step1006 and the write command processed at step 1008 by the receiving databank 110.

Here, processing the write command may entail that the parity segmentstored at the receiving data bank 110 may be updated based upon thewrite command. This update of the parity segment may be accomplished ina variety of ways, not all of which will be elaborated on herein butwhich will be known to those of ordinary skill in the art. For example,distributed RAID application 210 on parity data bank 110 may perform abacked out write in order to update the parity segment. Performing thisbacked out write may entail obtaining data segments from which theparity segment and performing logical operations (such as exclusive OR(XOR) operations) using the obtained data segments and the data to bewritten associated with the WRITE command. Alternatively, if distributedRAID application 210 on receiving data bank 110 has multiple WRITEcommands corresponding to each of the data segments from which theparity segment was created, a new parity segment may be calculated andthe original parity segment may be updated by replacing it with thenewly calculated parity segment. Other methods for updating the paritysegment may be realized from a review of the disclosures herein and theparticular method utilized to update a parity segment by a distributedRAID application may depend on a variety of factors, includingconfiguration parameters, the availability of certain data (for example,WRITE commands corresponding to all data segments used to create theparity, etc.) or any of a number of other factors.

Returning now to step 962, if the WRITE command was not received from ahost this may indicate that the WRITE command was received from anotherdata bank 110 (which, in many cases, may have been the data bank 110which originally received the WRITE command from a host 102). In thiscase, the WRITE command may be placed in the write cache of thereceiving data bank 110 at step 990 and a completion notification sentto the issuing data bank at step 992. The WRITE command may then beprocessed at step 994 (for example, the parity segment may be updated asdiscussed above).

After reviewing the above discussion it will be noted that in manycases, a distributed RAID application 210 at a particular data bank 110may not be able to process a received WRITE command until notificationis received from a parity data bank 110, that a parity data bank mayneed to evaluate multiple received WRITE commands to determine orimplement a method for updating the parity or any of a number of otherinstances when it may be desired to store one or more WRITE commands orevaluate a set of these stored WRITE commands. To facilitate the storageand evaluation of WRITE (or other) commands, each distributed RAIDapplication 210 may have an associated write cache 260.

A representation of one embodiment of a write cache is depicted in FIG.10. As WRITE commands are received by distributed RAID application 210they are placed in write cache 1100. Each of these WRITE commands 1160may have an associated timestamp indicating when the WRITE command 1160was received. Thus, in one embodiment write cache 1100 may comprise aqueue of time stamped WRITE commands 1160. At some point a timestampmarker may be issued by distributed RAID application 210. This timestampmarker may comprise a time and be communicated to each of distributedRAID applications 210. When to issue a timestamp market may bedetermined in a variety of ways, such as when the write cache 1100 is acertain percentage full or when a certain number of WRITE commands 1160have been received, at a certain time interval or a variety of othermethodologies.

In any event, this timestamp marker 1110 will segment each of the writecaches 1110 associated with each of the distributed RAID applications210 into at least two segments a closed marker 1150 comprising WRITEcommands 1160 received before the timestamp marker 1110 (in this exampleWRITE commands 1160 a, 1160 b, 1160 c and 1160 d) and an open marker1140 comprising WRITE commands 1160 received after the timestamp marker1110 (in this example WRITE commands 1160 e, 1160 f and 1160 g).Distributed RAID application 210 may then evaluate the set of WRITEcommands 1160 in the closed marker 1150 (in this example WRITE commands1160 a, 1160 b, 1160 c and 1160 d) to determine how these WRITE commands1160 are to be processed while received WRITE commands may still beadded to open marker 1140.

Conversely, as the closed marker 1150 comprises a set of WRITE commandswhich are no longer changing distributed RAID application may evaluatethis set of WRITE commands 1160 with respect to one another (or othercriteria) to determine an order of execution (and may therefore reorderWRITE commands 1160 in closed marker 1160), a methodology to update aparity segment (for example, if there are WRITE commands in closedmarker 1150 which correspond to each data segment used to create aparity) or make other determinations associated with the processing ofWRITE commands 1160. It will be noted that as a timestamp marker 1110may be issued for multiple reasons by any of distributed RAIDapplications 210 on each of data banks 110, multiple closed markers mayexist at any one point, for example, when multiple timestamp markers1110 are issued by distributed RAID applications 210 between the timethe write cache is evaluated by any one of the distributed RAIDapplications 210.

After reviewing the above the reader may now have an understanding ofhow distributed RAID applications 210 on data banks 110 operate intandem to achieve virtualized storage and RAID implementation. It may befurther helpful to an understanding to certain embodiments, however, todiscuss the functioning of certain embodiments of distributed RAIDapplication 210 after the occurrence of a fault. As discussed above,distributed RAID application 210 may be aware (for example, have stored)of a data bank which is faulty (in other words, which may have ahardware, software, communication or other fault which impedes orhampers the ability of the data bank 110 to operate or access data).Distributed RAID application 210 may be able to account for such faultswhile satisfying commands from hosts 102.

To illustrate, FIG. 11 depicts a flow diagram for one embodiment of amethod for implementing a READ command in a distributed RAID system. Itwill be understood that this method applies to an embodiment where RAID5 has been implemented in conjunction with a range of a volume and thatother embodiments may be equally well applied in cases where otherlevels (or no level) of RAID have been implemented. The READ command maybe sent by a host 102 to a data bank 110 through a switch 120 or fromanother data bank 110 to the receiving data bank 110. At step 1110,then, a READ command may be received at a data bank 110. The distributedRAID application 210 on data bank 110 may determine, at step 1220, adata segment which corresponds to a logical address referenced in thereceived READ command and on which data bank 110 the data segment isstored at step 1230. If the data bank 110 which is storing the datasegment is the same as the data bank 110 which received the READ command(as determined at step 1240) the requested data can be obtained from theappropriate disk 252 of the data store 250 on the receiving data bank110 at step 1250 and the received READ command responded to using theobtained data at step 1252. If the receiving data bank 110 received theREAD command from the host 102 the host 102 may be responded to while ifthe receiving data bank 110 received the READ command from another databank 110 the response may be sent to the distributed RAID application210 on the data bank 110 which issued the READ command.

If, however, the data segment is stored on a remote data bank 110 (adata bank 110 other than the one which received the command) at step1254 it may be determined if the remote data bank 110 on which the datasegment to be read is stored has experienced a fault. If not the READcommand may be sent to the distributed RAID application at the remotedata bank 110. After a response comprising the requested data isreceived from the remote data bank 110 at step 1258 the received READcommand may be responded to using that data at step 1260.

If the remote data bank 110 has experienced a fault, however, it may bedetermined at step 1254 if the receiving data bank 110 holds the paritysegment corresponding to the data segment associated with the READcommand. If the parity segment is stored at the receiving data bank 110the data segment corresponding to the READ command may be obtained usingthe parity segment stored at the receiving data bank 110. Obtaining thedata segment from the parity data may be accomplished in a variety ofway which will not be elaborated on in more detail, including obtainingthe other data segments (data segments other than the one correspondingto the READ command) from other data banks 110 and obtaining the desireddata segment by performing logical operations between the other datasegment and the parity segments. Once the requested data segment hasbeen obtained using the parity data at step 1268 the received READcommand may be responded to at step 1270. If the receiving data bank 110received the READ command from the host 102 the host 102 may beresponded to while if the receiving data bank 110 received the READcommand from another data bank 110 the response may be sent to thedistributed RAID application 210 on the data bank 110 which issued theREAD command.

If the receiving data bank 110 is not the data bank 110 storing theparity block a READ command may be sent to the remote data bank 110 onwhich the parity segment corresponding to the data segment referenced inthe READ command is stored at step 1262. After a response comprising therequested data is received from the remote parity data bank 110 at step1264 the received READ command may be responded to using that data atstep 1266.

Moving on to FIG. 12, a flow diagram for one embodiment of a method forimplementing a WRITE command in a distributed RAID system is depicted.This WRITE command may be sent by a host 102 to a data bank 110 througha switch 120 or from one data bank 110 to another data bank 110. At step1310, then, a WRITE command may be received at a receiving data bank110. The distributed RAID application 210 on receiving data bank 110 maythen determine at steps 1312, 1314 and 1320 the data segmentcorresponding to a logical address referenced by the WRITE command, thelocation of that data segment (for example, which data banks 110 isstoring the data corresponding to that segment) and the location of theparity segment corresponding to that data segment (for example, whichdata bank 110 is stores the segment where parity data created from thedata corresponding to that segment is stored). As discussed above, thelocation of both the data segment and the parity segment may bedetermined using the set of tables 240 stored on the receiving data bank110.

It can then be determined if either the data bank 110 on which the datasegment is stored or the data bank 110 on which the parity segment isstored have experienced a fault. If neither of those data banks 110 hasexperience a fault a normal write operation may be carried out at step1324 by the distributed RAID application. A normal write operation hasbeen discussed previously with respect to FIGS. 9A and 9B and will notbe discussed further.

If however, either of those data banks 110 has experienced a fault awrite operation taking into consideration the failed data bank 110 maybe conducted at step 1326. This write operation may parallelsubstantially identically the write operation described with respect toFIGS. 9A and 9B with the exception that before a write command is sentto a data bank 110 it is determined if the data bank 110 to which thewrite is to be sent is failed and if so that write command is not sentto the failed data bank 110. In all other respects the write operationis substantially identical.

After reviewing the aforementioned flow diagrams the operation ofcertain embodiments may be better understood with reference to specificexamples of one embodiment of a distributed RAID system in operation. Tothat end, attention is directed back to FIG. 4 which illustrates oneembodiment of a distributed RAID system with five data banks 110. Tobegin with a first example, suppose that host 102 b issues a READcommand to data bank 110 c, where the READ command references a logicaladdress which corresponds to data segment “2” 402 c on data bank 110 a.Here, distributed RAID application 210 c on data bank 110 c maydetermine that the logical address of the received READ commandreferences data segment “2” 402 c and that data segment “2” 402 c isstored on data bank 110 a. Distributed RAID application 210 c may thensend a corresponding READ command to data bank 110 a.

Distributed RAID application 210 a on data bank 110 a may receive thisREAD command, determine that the READ command references a logicaladdress which corresponds to data segment “2” 402 c and that datasegment “2” 402 c is located on the data bank 110 a on which it isexecuting. Distributed RAID application 210 a may then access data store250 a to obtain the data requested from data segment “2” 402 c andreturn this obtained data to the distributed RAID application 210 c atissuing data bank 110 c. Distributed RAID application 210 c on data bank110 c may receive this response from distributed RAID application 210 aon data bank 110 a and use data from this response to respond to theoriginal READ command issued from host 102 b.

Now suppose that host 102 b issues a READ command to data bank 110 c,where the READ command references a logical address which corresponds todata segment “2” 402 c on data bank 110 a, but that data bank 110 a hasexperience a fault and is no longer operating. In this case, distributedRAID application 210 c on data bank 110 c may determine that the logicaladdress of the received READ command references data segment “2” 402 cand that data segment “2” 402 c is stored on data bank 110 a.Additionally, distributed RAID application 210 c on data bank 110 c mayalso determine that data bank 110 a has experienced a fault.

Accordingly, distributed RAID application 210 c may determine that thelocation of parity segment 402 j corresponding to data segment “2” 402 cis data bank 110 e. Distributed RAID application 210 c may then send aREAD command to data bank 110 e. Distributed RAID application 210 e ondata bank 110 e may receive this READ command, determine that the READcommand references a logical address which corresponds to data segment“2” 402 c and that the parity segment 402 j corresponding to datasegment “2” 402 c is located on the data bank 110 e on which it isexecuting. Distributed RAID application 210 e may then access data store250 e to access parity segment 402 j and obtain the data requested fromdata segment “2” 402 c using the parity segment 402 j. This obtaineddata may be returned to the distributed RAID application 210 c atissuing data bank 110 c. It will be noted that distributed RAIDapplication 210 e may need other data to determine the data requestedfor data segment “2” 402 c. Accordingly, distributed RAID application210 e may determine that the location of data segment “0” 402 a, datasegment “1” 402 b and data segment “3” 402 d which were used inconjunction with data segment “2” 402 c to create parity segment 402 jare located respectively on data banks 110 b, 110 d and 110 c.Distributed RAID application 210 e may thus obtain data segment “0” 402a, data segment “1” 402 b and data segment “3” 402 d by sending READrequests to these data banks 110 b, 110 d and 110 c and use data segment“0” 402 a, data segment “1” 402 b and data segment “3” 402 d inconjunction with parity segment 402 j to obtain the data requested fromdata segment “2” 402 c.

Distributed RAID application 210 c on data bank 110 c may receive theresponse from distributed RAID application 210 e on data bank 110 e anduse data from this response to respond to the original READ commandissued from host 102 b. In this manner, data corresponding to a datasegment can still be read by a host despite the occurrence of a fault inthe distributed RAID system.

Continuing on with WRITE commands, suppose that host 102 b issues aWRITE command to data bank 110 c, where the WRITE command references alogical address which corresponds to data segment “2” 402 c on data bank110 a. Here, distributed RAID application 210 c on data bank 110 c maydetermine that the logical address of the received WRITE commandreferences data segment “2” 402 c and that data segment “2” 402 c isstored on data bank 110 a. Furthermore, distributed RAID application 210c may determine that the parity segment 402 j corresponding to datasegment “2” 402 c is located on data bank 110 e. Distributed RAIDapplication 210 c may then send a corresponding WRITE command to databanks 110 a and 110 e. Upon receiving completion notifications fromdistributed RAID applications 210 a and 210 e, distributed RAIDapplication 210 c may respond to the originally received WRITE command.

Distributed RAID application 210 e on data bank 110 e may receive itscorresponding WRITE command, determine that the WRITE command referencesa logical address which corresponds to data segment “2” 402 c and thatthe parity segment 402 j corresponding to data segment “2” 402 c islocated on the data bank 110 e on which it is executing. DistributedRAID application 210 e may place the WRITE command in its write cacheand send a completion notification to data bank 110 c. Distributed RAIDapplication 210 e may then access data store 250 e to access paritysegment 402 j and update the parity segment 402 j using the datareferenced in the received WRITE command.

Distributed RAID application 210 a on data bank 110 a may receive itscorresponding WRITE command, determine that the WRITE command referencesa logical address which corresponds to data segment “2” 402 c and thatdata segment “2” 402 c is located on the data bank 110 a on which it isexecuting. Distributed RAID application 210 a may place the WRITEcommand in its write cache and send a completion notification to databank 110 c. Distributed RAID application 210 a may then access datastore 250 a to update the segment “2” 402 c using the data referenced inthe received WRITE command.

Again suppose now that host 102 b issues a WRITE command to data bank110 c, where the WRITE command references a logical address whichcorresponds to data segment “2” 402 c on data bank 110 a, but that databank 110 a has experience a fault and is no longer operating. In thiscase, distributed RAID application 210 c on data bank 110 c maydetermine that the logical address of the received WRITE commandreferences data segment “2” 402 c and that data segment “2” 402 c isstored on data bank 110 a. Additionally, distributed RAID application210 c on data bank 110 c may also determine that data bank 110 a hasexperienced a fault. Furthermore, distributed RAID application 210 c maydetermine that the parity segment 402 j corresponding to data segment“2” 402 c is located on data bank 110 e. Distributed RAID application210 c may then send a corresponding WRITE command to data bank 110 e.Upon receiving a completion notification from distributed RAIDapplications 210 e distributed RAID application 210 c may respond to theoriginally received WRITE command.

Distributed RAID application 210 e on data bank 110 e may receive thecorresponding WRITE command, determine that the WRITE command referencesa logical address which corresponds to data segment “2” 402 c and thatthe parity segment 402 j corresponding to data segment “2” 402 c islocated on the data bank 110 e on which it is executing. DistributedRAID application 210 e may place the WRITE command in its write cacheand send a completion notification to data bank 110 c. Distributed RAIDapplication 210 e may then access data store 250 e to access paritysegment 402 j and update the parity segment 402 j using the datareferenced in the received WRITE command. In this manner, datacorresponding to a data segment can still be written by a host despitethe occurrence of a fault in the distributed RAID system.

In the foregoing specification, the invention has been described withreference to specific embodiments. However, one of ordinary skill in theart appreciates that various modifications and changes can be madewithout departing from the scope of the invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofinvention.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any component(s) thatmay cause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or component of any or all the claims.

1. A system for implementing distributed RAID, comprising: one or morehosts; a plurality of data bank computers, each data bank coupled toeach host and each of the other plurality of data banks, each data bankincluding: a processor; a data store; a computer readable mediumcomprising instructions executable to: receive a command correspondingto a first segment of a volume from a first host of the one or morehosts, wherein the volume comprises a set of segments, and the set ofsegments of the volume are stored on the plurality of data banksaccording to a data bank order, where the data bank order is apermutation of the plurality of data banks and redundancy datacorresponding to a RAID level implemented in conjunction with the volumeis stored on each of the plurality of data banks, such that for eachsegment of the volume a corresponding redundancy segment comprising theredundancy data associated with that segment of the volume does notreside on the same data bank as the corresponding segment of the volume;determine a first data bank of the plurality of data banks, wherein thefirst data bank corresponds to the first segment; and if the first databank is the data bank which received the command execute the firstcommand with respect to the first segment and if not, forward the firstcommand to the first data bank.
 2. The system of claim 1, wherein thecomputer instructions are further executable to determine if the firstcommand is a write command and if the first command is a write commanddetermining a second data bank of the plurality of data banks, whereinthe second data banks corresponds to a first redundancy segmentassociated with the first segment and if the second data bank is thedata bank which received the command execute the first command withrespect to the first redundancy segment and if not, forward the firstcommand to the second data bank.
 3. The system of claim 2, whereindetermining the first data bank and determining the second data bankcomprises accessing a global table in the data store, the global tablecomprising the permutation of the data banks and the RAID level.
 4. Amethod for distributed RAID implemented on a computing device having aprocessor, comprising: establishing a volume on a plurality of databanks, where the volume comprises a set of segments and the set ofsegments of the volume are stored on the plurality of data banksaccording to a data bank order, where the data bank order is apermutation of the plurality of data banks; implementing a RAID level inconjunction with the volume, wherein implementing the RAID levelcomprises storing a set of redundancy segments, each redundancy segmentcorresponding to one or more of the set of segments of the volume suchthat each redundancy segment does not reside on the same data bank asthe corresponding one or more of the set of segments of the volume;receiving a command corresponding to a first segment of the set ofsegments of the volume; determining a first data bank of the pluralityof data banks, wherein the first data bank corresponds to the firstsegment; and executing the first command with respect to the firstsegment, wherein the first command is executed on the first data bank.5. The method of claim 4, further comprising determining if the firstcommand is a write command and if the first command is a write commanddetermining a second data bank of the plurality of data banks, whereinthe second data banks corresponds to a first redundancy segmentassociated with the first segment and executing the first command withrespect to the first redundancy segment, wherein the first command isexecuted on the second data bank.
 6. The method of claim 5, whereindetermining the first data bank and determining the second data bankcomprises accessing a global table comprising the permutation of thedata banks and the RAID level.
 7. A method for distributed RAIDimplemented on one or more data bank computers, each data bank computerscoupled to one or more hosts and each of the other plurality of databanks, comprising: receiving a command corresponding to a first segmentof a volume from a host, wherein the volume comprises a set of segments,and the set of segments of the volume are stored on the plurality ofdata banks according to a data bank order, where the data bank order isa permutation of the plurality of data banks and redundancy datacorresponding to a RAID level implemented in conjunction with the volumeis stored on each of a plurality of data banks, such that for eachsegment of the volume a corresponding redundancy segment comprising theredundancy data associated with that segment of the volume does notreside on the same data bank as the corresponding segment of the volume;determining a first data bank of the plurality of data banks, whereinthe first data bank corresponds to the first segment; executing thefirst command with respect to the first segment, wherein the firstcommand is executed on the first data bank.
 8. The method of claim 7,further comprising determining if the first command is a write commandand if the first command is a write command determining a second databank of the plurality of data banks, wherein the second data bankscorresponds to a first redundancy segment associated with the firstsegment and executing the first command with respect to the redundancysegment, wherein the first command is executed on the second data bank.9. The method of claim 8, wherein determining the first data bank anddetermining the second data bank comprises accessing a global tablecomprising the permutation of the data banks and the RAID level.
 10. Acomputer readable medium comprising computer executable instructions forimplementing distributed RAID, the computer instructions executable for:establishing a volume on a plurality of data banks, where the volumecomprises a set of segments and the set of segments of the volume arestored on the plurality of data banks according to a data bank order,where the data bank order is a permutation of the plurality of databanks; implementing a RAID level in conjunction with the volume, whereinimplementing the RAID level comprises storing a set of redundancysegments, each redundancy segment corresponding to one or more of theset of segments of the volume such that each redundancy segment does notreside on the same data bank as the corresponding one or more of the setof segments of the volume; receiving a command corresponding to a firstsegment of the set of segments of the volume; determining a first databank of the plurality of data banks, wherein the first data bankcorresponds to the first segment; and executing the first command withrespect to the first segment, wherein the first command is executed onthe first data bank.
 11. The computer readable medium of claim 10,wherein the instructions are further executable for determining if thefirst command is a write command and if the first command is a writecommand determining a second data bank of the plurality of data banks,wherein the second data banks corresponds to a first redundancy segmentassociated with the first segment and executing the first command withrespect to the first redundancy segment, wherein the first command isexecuted on the second data bank.
 12. The computer readable medium ofclaim 11, wherein determining the first data bank and determining thesecond data bank comprises accessing a global table comprising thepermutation of the data banks and the RAID level.
 13. A computerreadable medium comprising computer executable instructions forimplementing distributed RAID a plurality of data bank computers, thecomputer instructions executable for: receiving a command correspondingto a first segment of a volume from a host, wherein the volume comprisesa set of segments, and the set of segments of the volume are stored onthe plurality of data banks according to a data bank order, where thedata bank order is a permutation of the plurality of data banks andredundancy data corresponding to a RAID level implemented in conjunctionwith the volume is stored on each of a plurality of data banks, suchthat for each segment of the volume a corresponding redundancy segmentcomprising the redundancy data associated with that segment of thevolume does not reside on the same data bank as the correspondingsegment of the volume; determining a first data bank of the plurality ofdata banks, wherein the first data bank corresponds to the firstsegment; executing the first command with respect to the first segment,wherein the first command is executed on the first data bank.
 14. Thecomputer readable medium of claim 13, wherein the instructions arefurther executable for determining if the first command is a writecommand and if the first command is a write command determining a seconddata bank of the plurality of data banks, wherein the second data bankscorresponds to a first redundancy segment associated with the firstsegment and executing the first command with respect to the redundancysegment, wherein the first command is executed on the second data bank.15. The computer readable medium of claim 14, wherein determining thefirst data bank and determining the second data bank comprises accessinga global table comprising the permutation of the data banks and the RAIDlevel.